This invention relates generally to protective coatings for plastic articles and to methods for curing such coatings.
Engineering resins are well-known, commercially available materials possessing physical and chemical properties which are useful in a wide variety of applications. For example, polycarbonate has replaced glass in many products because of its excellent breakage resistance. Exemplary products made from polycarbonates include automobile headlamps and stoplight lenses; safety shields in windows, architectural glazing, and the like. However, a major defect exhibited by plastics such as polycarbonates is their very low scratch-resistance.
Scratch-resistant coatings for plastics have been available in the prior art. As an example, H. Clark discloses a protective coating composition for transparent plastics in U.S. Pat. No. 4,027,073. These coatings contain a dispersion of colloidal silica in a lower aliphatic alcohol-water solution of the partial condensate of a silanol. The composition is maintained at a pH in the range of 3.0 to 6.0.
As another example, B. Anthony, in U.S. Pat. No. 4,624,870, discloses novel silica-containing coating compositions having an alkaline pH and stabilized by the use of a base which is volatile at curing temperatures.
In U.S. Pat. No. 4,159,206, Armbruster et al. describe colloidal silica-based coatings which use a mixture of dialkyldialkoxysilanes and alkyltrialkoxysilanes.
In an improvement to the coatings described by Armbruster et al., G. Patel teaches a silica-containing curable coating composition containing a silane mixture and ammonium-stabilized colloidal silica having an average particle diameter no greater than about 5-10 microns, as described in application Ser. No. 185,594, filed Apr. 25, 1988, and assigned to the assignee of the presently-described invention.
As disclosed in the Patel application, the cure of silane-based protective coatings is enhanced by the use of a cure catalyst. Many types of cure catalysts are available in the art, as shown by some of the above-mentioned disclosures. However, recent movement in the industry toward greater processing efficiency in preparing coated articles such as glazing has created the need for much better cure performance. Such performance can be measured by the achievement of lower cure temperatures or shorter cure times. The catalyst must still fulfill other requirements, such as not adversely affecting the weathering properties of the coating after cure.
It is therefore an objective of the present invention to provide an improved method for curing colloidal silica-based silicone coating compositions.
It is another objective of this invention to provide a highly effective cure catalyst for colloidal silica-based silicone coating compositions which does not adversely affect any of the properties of the composition after cure.
It is yet another objective of this invention to provide silicone-based curable coating compositions which are characterized by high cure rates.